This application pertains to the art of machine vision and more particularly to high speed automated video inspection. The invention is particularly applicable to automated video inspection of continuous web-like materials such as cloth, paper, MYLAR, sheet metal, etc., and will be described with particular reference thereto, although it will be appreciated that the invention has broader applications such as in the inspection of any continuously moving specimen whether discrete or continuous in which the specimen passes through the field of view of an associated inspection camera and in systems utilizing relatively low illumination levels.
Machine vision systems have obtained an established presence in industry to accomplish high speed video inspections. Such machine vision systems are generally comprised of a lighting system to illuminate a specimen and a camera for sensing light reflected therefrom. A digitized image is formed from an image received by the camera. Data representative of this image is then utilized for determining acceptability of the specimen in view of preselected physical characteristics thereof.
Earlier array video inspection systems were typically geared to inspection of a continuous sequence of generally uniform specimens which could be contained within the field of view of the inspecting camera. These systems employed lighting which was sufficient to allow for a single illumination period. Still other earlier systems employed indexed cameras which are progressively incremented relative to a subportion of a large, usually planar, specimen to obtain a series of images thereof.
Substantial product is manufactured as a continuous stream of webbed or sheet-like material. While the aforementioned systems are adequate for a number of inspections, they provide no means for acquiring a consistently detailed inspection image of a continuous stream of fast moving web material. Earlier attempts to achieve automated inspection of such materials relied upon line scan cameras with continuous illumination. Stroboscopic systems were also utilized but required intense illumination periods. It was therefore desirable that a system be provided which allows for detailed high speed video inspection on a continuous stream of web material or which utilizes heretofore inadequate lighting intensities with improved image integrity and which exhibits robustness over a wide range of specimens.
More recently, advances in cameras, and particularly charge coupled device ("CCD") cameras, has led to time delay integration ("TDI"), techniques such as described by U.S. Pat. Nos. 4,922,337 and 4,949,172. TDI employs a CCD array in which rows of CCD elements which are arranged perpendicularly in relation to a direction of propagation of a continuous webbing or other specimens. A continuous light source reflects light from a generally linear cross-section of the specimen to a row of CCD elements. The resultant image data on that row is shifted to a subsequent, parallel row of elements in the CCD array, where additional light flux reflected from the same cross-section of the specimen is integrated therewith. Accordingly, low-light influence due to a single cross-section of the specimen is repeatedly obtained. The resulting combined image averages away substantial noise constituents providing an improved signal-to-noise ratio in a captured image. This allows for obtaining a continuous series of high integrity linear images across the webbing or other specimens.
While the aforementioned TDI technique provides a substantial improvement, it nonetheless presents certain disadvantages. As with more conventional video inspection systems, TDI inspection techniques center on numeric processing, rather than lighting technique. Previous techniques are conducive to some "smearing" of each linear cross-section image. Also, often times different grades of webbing or even entirely different webbing materials may at various times be inspected by the same system. Similarly, non-webbing systems often encounter markedly different specimens at different times. Differences in reflectivity in these situations require compensation. This is typically accomplished by compensation in the inspection algorithm software. Even this is limited given that absolute light sensitivity limits are inherent in CCDs, and once a sensitivity threshold has been exceeded, information is lost and compensation is not possible.
It is also possible to vary lighting intensity, with conventional lighting, however color temperature shifts inherent with incandescent sources and stringent frequency or current controls to effect modification of fluorescent sources are difficult and expensive.
Still more recently, substantial advances have been made in connection with light emitting diode technology. First generation light emitting diodes provided a high portion of their output in the infrared spectrum, which is to be expected from the physical characteristics of the semiconductor substrate from which they were fabricated. Since then, different semiconductor substrates have been utilized to generate light having wavelength frequency that is higher than infrared. Expectedly, first visible LEDs were primarily red in color.
As different substrates were utilized to form different LED colors, another physical property was manifested. It will be recalled that total light energy is related to a frequency by Planck's constant by the equation: EQU E=h.nu.
While this is of marginal concern in visual displays, where LEDs have and continue to find their dominant application, it is even more critical when LEDs are used for illumination. That is, when frequencies other than red are utilized, the total light energy available for inspection illumination is correspondingly lessened due to the frequency increase.
Further, charge-coupled devices are also fabricated in silicon. Accordingly, their maximum sensitivity is in the infrared area of the spectrum, analogously to the maximum spectral light output of silicon. Accordingly, first generation video illumination, utilizing LEDs, was with infrared lighting. Second generation lighting employed visible red LEDs. Recent advances and LEDs have provided other colors with a usable amount of energy. Further, advances in CCD technology have allowed for increased sensitivity to different illumination spectra.
The present invention contemplates a new and improved TDI video inspection and engineered lighting system which overcomes all the above-referred problems, and others, and provides a video inspection system allowing for continuous inspection of a stream of web materials or other specimens with improved integrity utilizing multiple or selectable color spectra.